This invention relates generally to optical fibers and, in particular, to methods for fabricating optical fiber preforms suitable for use as multi-mode optical fiber precursors.
The fabrication of an optical fiber, whether single-mode or multi-mode, begins with a preform. The preform is a silica-based structure that typically has a doped core region. The preform is then xe2x80x9cpulledxe2x80x9d to form the much thinner optical fiber.
The techniques for the fabrication of multi-mode optical fiber preforms have generally followed the processes used in single mode optical fiber preform fabrication. These techniques, and their drawbacks, are explained in the following.
MCVD (Modified Chemical Vapor Deposition) is an internal process in which many layers are added to form a graded refractive index (ideally with a parabolic profile in the refractive index.) This ideal profile for a multi-mode fiber is schematically illustrated in FIG. 1. It is this profile that provides the required high bandwidth fiber, with the bandwidth being significantly larger than that of a multi-mode step index fiber. However, current and future Local Area Networks (LANs) will require even wider bandwidths and, as a result, the step index, or rod in tube techniques, for fabricating preforms will not be adequate. If a single mode fiber is desired, a MCVD preform can be made, and the preform can be sleeved with a large thick walled tube. This permits many hundreds of kilometers of optical fiber to be pulled from a single MCVD preform, and thus makes MCVD competitive with other processes to be described below. However, this sleeving cannot be used for multi-mode fibers, since the multi-mode fiber core is typically significantly larger than the core of a single mode fiber, e.g., 50 microns to 62.5 microns for a 125 micron outer diameter, which can be an order of magnitude larger that the single mode fiber core. Such a large core cannot be built up with a sleeving that is designed to increase the amount of fiber pulled from a single, larger diameter preform.
OVD (Outside Vapor Deposition) is a process by which silicon tetrachloride (or, more recently, the vapors of a suitable organic precursor as described in U.S. Pat. No. 5,043,002 entitled xe2x80x9cMethod of Making Fused Silica by Decomposing Siloxanesxe2x80x9d to Dobbins et al.) and oxygen are combusted to form silica soot that impinges on a slightly tapered mandrel. The soot deposition builds up on this xe2x80x9cbait rodxe2x80x9d, and there is no limitation on the diameter of the final soot body. The xe2x80x9cbait rodxe2x80x9d, typically a tapered alumina rod, is removed, and the soot body is then sintered in a chlorine environment to form a vitreous, pore-free preform. The preform is then stretched to achieve the desired geometry and subsequently pulled into an optical fiber. This has proven to be a very successful method for the fabrication of single mode fibers that require only a small amount of germanium dioxide in the core of the fiber, and large amounts of lightly doped silica around the core. However, and as was discussed above, for a graded index multi-mode fiber an ideally parabolic profile is required to achieve a wide bandwidth.
Since it is possible, in principle, to make large core soot bodies using outside deposition, one might think that this is an ideal technique for the low cost manufacture of low loss, high bandwidth multi-mode fiber preforms. Unfortunately, this is not the case for the following reason. Even though a proper radial gradation in the germanium content of an unsintered soot boule can be achieved; in the sintering process, especially in the presence of chlorine, the germanium dioxide undergoes a reaction to form the germanium oxide, which has a considerable vapor pressure at the sintering temperatures that are required to form an amorphous preform. Thus, the needed parabolic gradation in the refractive index cannot be achieved. This is because of the fact that germanium is used, and the germanium chemistry destroys the incorporation of germanium dioxide at high temperatures.
It is noted that the use of non-chlorine containing precursors is described in U.S. Pat. No. 4,501,602 to Miller et al., wherein the precursors used in a vapor phase process do not contain chlorine.
Vapor Axial Deposition (VAD) is another technique for the fabrication of multi-mode preforms, and has the advantage that large soot boules can be made. However, with germanium dioxide as the index raising component it is difficult to maintain the desired parabolic profile during sintering, as was described previously for the OVD case.
It should be noted that in all of the fiber fabrication processes, discrete layers of doped glass are deposited. Since Ge is a glass former incorporated into the silica network, it does not readily diffuse, even at high temperatures. Thus, the mechanism of diffusion, which would smooth the boundaries between layers, is absent.
A brief reference is also made to a relatively new technology that is based on plastic fiber. However, this process is not yet technically mature.
Another current problem in the industry is an inability to fabricate up-doped silica preform tubes for cladding pumped optical fiber lasers.
It is a first object and advantage of this invention to provide an improved method for fabricating a preform suitable for use as a multi-mode optical fiber precursor.
It is a second object and advantage of this invention to provide a method to fabricate an up-doped silica preform tube for use in making a cladding pumped optical fiber laser.
It is another object and advantage of this invention to provide an improved method for fabricating a preform suitable for use in manufacturing a wide bandwidth multi-mode optical fiber, the method employing a liquid phase spray pyrolysis technique for generating silica soot at a high rate, in combination with non-chlorine containing liquid chemical precursors and a refractory, index of refraction raising additive that overcomes the problems inherent in the use of germanium-based chemistry at typical sintering temperatures.
It is one further object and advantage of this invention to provide a method to fabricate an all-glass cladding pumped optical fiber.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed for fabricating a preform suitable for use in manufacturing a wide bandwidth multi-mode optical fiber. The method includes steps of employing a liquid phase spray pyrolysis technique for generating silica soot at a high rate, in combination with a non-chlorine containing liquid silica precursor and a refractory, index of refraction raising additive that overcomes the problems inherent in the use of germanium-based chemistry at typical sintering temperatures. The refractory, index of refraction raising additive is preferably comprised of a Group V (Group VB) element oxide, such as a tantalum oxide. The liquid precursor is preferably comprised of a polymethylsiloxane, such as hexamethyldisiloxane, octamethylcyclotetrasiloxane (OMCCTS), or tetramethylcyclotetrasiloxane.
The step of employing a liquid phase spray pyrolysis technique includes the steps of: (a) merging at least two liquid streams, one comprised of the liquid silica precursor and another one comprised of the liquid silica precursor in combination with the index of refraction raising additive; (b) atomizing, spraying and then combusting the merged streams to form a flame that generates silica soot that contains the additive; and (c) varying the flow rate of at least one of the streams in order to vary a concentration of the additive in the silica soot.